1. Field of the Invention
This invention relates to an apparatus for solving logic and, more particularly, to a controller for solving boolean logic.
2. Background Information
Programmable logic controllers (PLCs) are well-known in the art. PLCs are utilized in a wide variety of industrial plants to solve ladder logic, such as a ladder diagram, which is represented by a power rail, plural input contacts, one or more output coils and a neutral rail. Typically, PLCs employ a combination of hardware and/or software circuits to rapidly solve the ladder logic.
Although ladder logic is readily understood by plant personnel responsible for the operation of an industrial plant, often, the control strategy, as designed by control engineers, is defined using standard graphic symbols. For example, when the control strategy includes boolean logic, circuit elements such as AND, OR and NOT logic gates are employed. Accordingly, whenever that control strategy is implemented in PLCs or other controllers for the plant control system, it is necessary to convert the control strategy from boolean logic to ladder logic.
It is known to solve either ladder logic or boolean logic in software. For example, a processor employs "hot code" which is pre-compiled to solve a particular set of ladder logic or boolean logic, respectively. Whenever changes to the ladder logic or boolean logic are required, it is necessary to recompile the "hot code" for the different logic configuration. Another disadvantage of "hot code" is that each of the respective ladder logic elements or boolean logic elements must be fully evaluated to determine the final logic outputs.
It is also known to monitor all inputs to the ladder logic or boolean logic for any change of state. Whenever a change of one or more inputs is detected, the corresponding logic is evaluated to determine the new results. Under normal conditions, during which only a relatively few inputs change over a relatively long period of time, there is no problem. However, under plant upset conditions (e.g., the trip of a power plant), many inputs may change in relatively short succession, thereby possibly overloading the control system.
Accordingly, there is room for improvement.